Hot dip Al coated steel sheet which is comprised of steel sheet on which an Al—Si alloy is coated has Si added to the coating layer, so maintains a silver white color and is excellent in heat reflection property even at a high temperature. For this reason, in the past, for example it has been used for mufflers for automobiles and various other heat resistant applications. However, it is known that this hot dip Al coated steel sheet also suffers from interdiffusion of Al and Fe when exposed to a high temperature of 450° C. or more, the Al—Si coating layer changes to an Al—Fe—Si-based intermetallic compound layer and discolors to a black color (hereinafter, unless otherwise indicated, this phenomenon being referred to as “alloying” or “black discoloration”, further, ease of black discoloration being referred to as a “black discoloration property”), loses gloss, and is remarkably impaired in heat reflection.
It is known that the amount of the solid solution nitrogen in the steel sheet is deeply involved in this alloying of the Al coating. In steel sheet which contains solid solution nitrogen in a certain amount or more, the fact that an AlN layer is formed at the interface of the alloy layer and the steel sheet and thereby an alloying reaction is suppressed is described in for example, Journal of the Iron and Steel Institute of Japan 70 (1984), S475 etc. Furthermore, it is known that by post annealing steel sheet which contains solid solution nitrogen, this AlN layer grows and, furthermore, the black discoloration temperature rises.
Based on this discovery, the art of suppressing black discoloration due to alloying has been studied in various ways up to now. For example, the applicants disclosed in PLT 1 an Al coated steel sheet giving heat black discoloration resistance by coating steel restricted in amounts of C, Si, N, Al, O, Ti, Nb, V, and B by hot dip Al coating, then annealing the coated steel sheet at 300 to 500° C. for 2 to 20 hours.
PLT 2 takes note of the fact that rimmed steel has a black discoloration temperature of 520° C. or so, while killed steel has a low one of 320° C., so focuses on the solid solution nitrogen (N) in the steel material of the killed steel and proposes a countermeasure. That is, it secures the solid solution nitrogen by restricting the Al and Ti which forms stable nitrides. For this reason, examples of methods of production of cast slabs for hot dip Al coated steel sheet use which define suitable ranges of C, Si, Mn, sol-Al, N, and O in order to obtain the effects of the deoxidation conditions etc. are disclosed.
PLT 3 proposes to coat a steel material of ingredients causing sol-N to stably remain by aluminum coating including a certain extent of Mg and then anneal it at 300 to 500° C. for 2 to 20 hours to form a monoclinic crystal structure of an Fe—Al—Si—Mg alloy between the steel sheet and the coating layer and, furthermore, to form AlN between the intermetallic compound and the steel material so as to prevent black discoloration due to interdiffusion of elements.
PLT 4 discovers that by complexly adding Mn and Cr to an aluminum coating layer and annealing after coating at 300 to 500° C. for 0.5 hour or more, these elements remarkably concentrate at the interface of the alloy layer and coating layer and discloses that this layer exhibits the effect of suppression of alloying. For this reason, it has been proposed that this exhibits the effect of improvement of retention of gloss.